I recommend cpanel+cloudlinux+litespeed as the web server management and optimization software.
Currently using a shared hosting with 1.5GB RAM and 2cpu +litespeed and works very well with 200 concurrent users.
Litespeed will boost your cpu/ram and allow super cache functionality.
Cloudlinux will limit resource usage for an account avoiding a server to come done completely.
Don't have experience with 5k concurrent users, but in your case I would go with a big vps (12cpu/32gb ram?) and cpanel software.
Currently using a shared hosting with 1.5GB RAM and 2cpu +litespeed and works very well with 200 concurrent users.
Litespeed will boost your cpu/ram and allow super cache functionality.
Cloudlinux will limit resource usage for an account avoiding a server to come done completely.
Don't have experience with 5k concurrent users, but in your case I would go with a big vps (12cpu/32gb ram?) and cpanel software.